1. Field of the Invention
The present invention is in the field of tools used for drilling oil and gas wells. Specifically, this invention applies to the drilling of a new well bore which branches off from an existing well bore which has been drilled and cased. This invention also applies to drilling through a cemented hole, followed by milling out a bridge plug or float equipment.
2. Background Information
It very often occurs that after a well bore has been drilled and the casing installed, a need arises to drill a new well bore off to the side, or at an angle, from the original well bore. The new well bore may be a lateral bore extending outwardly from the original vertical well bore. The process of starting a new well bore from the existing bore is often called "kicking off" from the original bore. Kicking off from an existing well bore in which metal casing has been installed requires that the casing first be penetrated at the desired depth.
Typically, a section mill or window mill is used to penetrate the metal casing, then the window mill and the drill string are withdrawn from the well bore. Following the milling of the window, a drill bit is mounted on the drill string, run back into the well, and used to drill the lateral well bore. Tripping in and out of the well bore delays the drilling process and makes the well more expensive to complete. The reason for using two different tools in spite of this is that the window mill must penetrate the metal casing, while the drill bit must penetrate the subterranean formation, which often contains highly abrasive constituents.
Similarly, when it is necessary to drill through a cemented hole, then mill away downhole metal items, two trips must be made. First, a drill bit is attached to the drill string, run into the hole, and used to drill through the cement. The drill string is then tripped out, the drill bit removed, and a milling tool is attached. The drill string is then run into the hole to mill away the bridge plug or other metal member.
Milling of metal requires a type of cutting insert which is formed of a material hard enough to cut the metal but durable enough to avoid excessive breakage or chemical deterioration of the insert. If the insert crumbles or deteriorates excessively, the insert will lose the sharp leading edge which is considered most desirable for the effective milling of metal. Both hardness and durability are important. It has been found that a material such as tungsten carbide is sufficiently hard to mill typical casing steel, while it is structurally durable and chemically resistant to exposure to the casing steel, allowing the insert to wear away gradually rather than crumbling, maintaining its sharp leading edge.
Drilling through a rock formation or cement requires a type of cutting insert which is formed of a material as hard as possible, to allow the insert to gouge or scrape chunks out of the rock or cement without excessive wear or abrasion of the insert. This permits the drilling operator to drill greater lengths of bore hole with a single drill bit, limiting the number of trips into and out of the well. It has been found that a material such as polycrystalline diamond is an excellent choice for drilling through a rock formation or cement, because of its extreme hardness and abrasion resistance.
Tungsten carbide is not as good as polycrystalline diamond for drilling through rock or cement, because the diamond is harder and will therefore last longer, limiting the number of trips required. Polycrystalline diamond is not as good for milling through metal casing as tungsten carbide, because the diamond is not as structurally durable, allowing it to crumble more readily and destroy the sharp leading edge. Further, polycrystalline diamond has a tendency to deteriorate through a chemical reaction with the casing steel. There is a chemical reaction between the iron in the casing and the diamond body, which occurs when steel is machined with a diamond insert. As a result of this chemical reaction, the carbon in the diamond turns to graphite, and the cutting edge of the diamond body deteriorates rapidly. This prevents the effective machining of the steel casing with diamond. Therefore, tungsten carbide is the better choice for milling through the metal casing, and polycrystalline diamond is the better choice for drilling through rock or cement.
Unfortunately, in both of these types of operations, use of each type of cutting insert in its best application requires that a first tool be used to perform a first operation, and that a second tool be used to perform a second operation. This means that two trips are required for the kickoff and drilling operation, or for the cement drilling and bridge plug milling operation. It would be very desirable to be able to perform a single trip operation, thereby eliminating at least one trip into and out of the bore hole.